1. Field of the Invention
The present invention relates to an aqueous emulsion having the advantages of good water resistance, high-temperature viscosity stability and low-temperature storage stability, and to a method for producing such an aqueous emulsion having the advantage of good polymerization stability.
2. Description of the Related Art
Heretofore, polyvinyl alcohol (hereinafter referred to as PVA) is widely used as protective colloid in emulsion polymerization of an ethylenic unsaturated monomer, especially a vinyl ester monomer such as typically vinyl acetate. Aqueous emulsions of vinyl ester polymers obtained through emulsion polymerization with PVA serving as protective colloid have many applications in various fields of adhesives for paper products, wood products, plastics, etc., binders for dip-coated paper, non-woven fabrics, etc., as well as admixtures, jointing agents, coating compositions, paper processing agents, fiber processing agents, etc.
Controlling the degree of hydrolysis of PVA therein makes it possible to change the physical properties of such aqueous emulsions, some of which will have low viscosity just like Newtonian fluid, and will be relatively highly resistant to water, while some others will have high viscosity relatively independently of ambient temperatures. Depending on such their properties, aqueous emulsions containing PVA have many applications in various fields.
However, some aqueous emulsions containing PVA have drawbacks in that their fluidity (high-speed coatability) is poor, that their water resistance is poor, that their viscosity greatly varies depending on ambient temperatures, and that their viscosity significantly increases at low temperatures. It is known that the properties of aqueous emulsions containing PVA greatly depend on PVA therein used for emulsion polymerization.
PVA serving as a dispersant for emulsion polymerization is generally grouped into xe2x80x9ccompletely hydrolyzed PVAxe2x80x9d having a degree of hydrolysis of 98 mol % or so and xe2x80x9cpartially hydrolyzed PVAxe2x80x9d having a degree of hydrolysis of 88 mol % or so. Aqueous emulsions containing the former PVA could have relatively good water resistance and fluidity (high-speed coatability), but are problematic in that their viscosity greatly increases and therefore they soon gel when left at low temperatures. On the other hand, aqueous emulsions containing the later PVA could have improved properties in some degree in that their viscosity increase at low temperatures is retarded and they are not so much gelled even at low temperatures. However, they are still problematic in that their water resistance is poor. To solve the problems with these aqueous emulsions containing PVA, using the two types of PVA as combined in aqueous emulsions, or using PVA having an intermediate degree of hydrolysis therein is tried. However, no one has heretofore succeeded in realizing aqueous, PVA-containing emulsions that satisfy the two requirements of good water resistance and good low-temperature storage stability for their viscosity. In that situation, PVA with ethylene units therein has been proposed for use in aqueous emulsions (see Japanese Patent Laid-Open Nos. 81666/1996, 80709/1994, 226774/1998, etc.). Containing it, aqueous emulsions have much improved water resistance and low-temperature storage stability. However, their high-temperature viscosity stability is not still good. In other words, their viscosity still varies, depending on ambient temperatures, and, in addition, their high-temperature storage stability is not also good.
The present invention is to solve the problems with the related art techniques, and its object is to provide an aqueous emulsion having the advantages of far improved high-temperature viscosity stability (that is, the emulsion is anywhere stable independently of ambient temperatures, and its high-temperature storage stability is good) and far improved water resistance and low-temperature storage stability, and also to provide a method for producing such an aqueous emulsion having the advantage of good polymerization stability.
Specifically, the invention provides an aqueous emulsion which comprises, as the dispersant, a polyvinyl alcohol containing from 1 to 15 mol % of ethylene units in the molecule and having a degree of hydrolysis of at least 95 mol %, and, as the dispersoid, a vinyl ester polymer, and which is characterized in that, when its film is immersed in water at 20xc2x0 C. for 24 hours, the degree of water solubility of the film is at most 1.5%, and the degree of water absorption of the film is at most 30%, and that the ratio of the emulsion viscosity at 60xc2x0 C. (T60xc2x0 C.) to the emulsion viscosity at 20xc2x0 C. (T20xc2x0 C.), T60xc2x0 C./T20xc2x0 C. is at most 2.
In this, the degree of emulsion release and the degree of water absorption are measured according to the methods mentioned hereinunder. Films having a degree of water solubility of them of at most 1.5% and having a degree of water absorption of at most 30% are meant to have good water resistance. Preferably, the degree of water solubility of the film obtained from the emulsion of the invention is at most 1.1%; and the degree of water absorption of the film is at most 27%.
The ratio of the emulsion viscosity at 60xc2x0 C. (T60xc2x0 C.) to the emulsion viscosity at 20xc2x0 C. (T20xc2x0 C.), T60xc2x0 C./T20xc2x0 C. is measured according to the method mentioned hereinunder. In general, ordinary vinyl ester polymer emulsions containing, as the dispersant, an ethylene-containing PVA have high viscosity at around 60xc2x0 C. However, the aqueous emulsion of the invention is so defined that its viscosity change ratio T60xc2x0 C./T20xc2x0 C. is at most 2. This means that the viscosity increase in the aqueous emulsion of the invention is retarded at around 60xc2x0 C., or that is, the temperature dependence of the aqueous emulsion of the invention is small. As its viscosity increase at around 60xc2x0 C. is thus retarded, the aqueous emulsion of the invention shall have the advantages of viscosity increase retardation anytime during polymerization to give the emulsion, and even during high-temperature storage, transportation and use of the emulsion. Accordingly, the workability and the handlability of the emulsion of the invention are much improved.
More preferably, the ratio of the emulsion viscosity after stored at 60xc2x0 C. for 1 week (H60xc2x0 C.) to that after stored at 20xc2x0 C. for 1 week (H20xc2x0 C.), H60xc2x0 C./H20xc2x0 C., is at most 2.5. The viscosity change ratio H60xc2x0 C./H20xc2x0 C. of the emulsion is measured according to the method mentioned hereinunder. The emulsion having a viscosity change ratio, H60xc2x0 C./H20xc2x0 C. of at most 2.5 is meant to have good high-temperature storage stability. Concretely, the viscosity of the emulsion having such a limited viscosity change ratio does not increase so much even when it is stored at high temperatures of around 60xc2x0 C. for 1 week or so, and the emulsion can be well worked and handled anytime at such high temperatures.
The emulsion of the invention can be prepared by polymerizing a vinyl ester monomer through emulsion polymerization in the presence of (1) a modified PVA containing from 1 to 15 mol % of ethylene units in the molecule and having a degree of hydrolysis of at least 95 mol %, which serves as a dispersant, and (2) at least one polymerization initiator selected from hydrogen peroxide, ammonium persulfate and potassium persulfate in a molar ratio to the vinyl ester monomer of from 0.001 to 0.01, in such a polymerization mode that (3) from 5 to 20% by weight of all the vinyl ester monomer is fed into the reactor in the initial stage of polymerization and the initiator is fed thereinto all at a time in a molar ratio to the. initial feed of the vinyl ester monomer of from 0.005 to 0.025.
The modified PVA containing from 1 to 15 mol % of ethylene units in the molecule and having a degree of hydrolysis of at least 95 mol %, which serves as the dispersant in the aqueous emulsion of the invention, is obtained by hydrolyzing a copolymer of vinyl ester and ethylene.
The vinyl ester includes, for example, vinyl formate, vinyl acetate, vinyl propionate, vinyl pivalate, etc. From its economical advantage, preferred is vinyl acetate.
Of the modified PVA, the ethylene unit content must fall between 1 and 15 mol %, but preferably between 3 and 13 mol %, more preferably between 5 and 12 mol %. In case where PVA having an ethylene unit content of smaller than 1 mol % is used, it is impossible to obtain an aqueous emulsion having good water resistance (to be indicated by the degree of emulsion release and the degree of water absorption) and good high-temperature viscosity stability, and, in addition, the low-temperature storage stability of the aqueous emulsion containing such PVA is poor, as so demonstrated in Comparative Example 1 to be mentioned hereinunder. On the other hand, PVA having an ethylene unit content of larger than 15 mol % is poorly soluble in water. Therefore, if such PVA is used, it will be impossible to obtain a stable aqueous emulsion.
Not interfering with the object of the invention, the dispersant may be copolymerized with ethylenic unsaturated comonomers. The ethylenic unsaturated comonomers include, for example, acrylic acid, methacrylic acid, fumaric acid, maleic acid (anhydride), itaconic acid, acrylonitrile, methacrylonitrile, acrylamide, methacrylamide, trimethyl(3-acrylamido-3-dimethylpropyl)ammonium chloride, acrylamido-2-methylpropanesulfonic acid and its sodium salt, ethyl vinyl ether, butyl vinyl ether, N-vinylpyrrolidone, vinyl chloride, vinyl bromide, vinyl fluoride, vinylidene chloride, vinylidene fluoride, tetrafluoroethylene, sodium vinylsulfonate, sodium allylsulfonate, etc.
As the case may be, also usable herein is a mercapto-terminated or carboxyl-terminated modified PVA which is obtained by copolymerizing a vinyl ester monomer such as vinyl acetate or the like with ethylene in the presence of a thiol compound such as thiolacetic acid, mercaptopropionic acid or the like, followed by hydrolyzing the resulting copolymer.
The degree of hydrolysis of the ethylene-modified PVA, which is used as the dispersant in the aqueous emulsion of the invention, must be at least 95 mol %, but preferably at least 96 mol %, more preferably at least 97 mol %. In case where PVA having a degree of hydrolysis of smaller than 95 mol % is used, it is impossible to obtain an aqueous emulsion having good water resistance. Preferably, the degree of polymerization the viscosity average polymerization degree) of the modified PVA for use herein falls between 100 and 3000, more preferably between 300 and 3000. PVA having a degree of polymerization of smaller than 100 will fail to exhibit the characteristic of protective colloid; but PVA having a degree of polymerization of larger than 3000 is problematic in its industrial-scale production.
The vinyl ester monomer to form the dispersoid in the aqueous emulsion of the invention includes, for example, vinyl formate, vinyl acetate, vinyl propionate, vinyl pivalate, etc. Vinyl acetate is preferred, as being economical.
In the invention, the monomer to form the dispersoid is mainly a vinyl ester monomer such as that mentioned above. However, combining such a vinyl ester monomer and ethylene is also preferred in the invention. Polymerizing the monomer(s) in a mode of emulsion polymerization gives an aqueous emulsion containing, as the dispersoid, a vinyl ester polymer or a vinyl ester-ethylene copolymer.
Not interfering with the object of the invention, the vinyl ester monomer may be copolymerized with any of ethylenic unsaturated monomers and dienic monomers. The comonomers include, for example, olefins such as propylene, isobutylene, etc.; halogeno-olefins such as vinyl chloride, vinyl fluoride, vinylidene chloride, vinylidene fluoride, etc.; acrylic acid and its esters such as acrylic acid, methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, dodecyl acrylate, 2-hydroxyethyl acrylate, etc.; methacrylic acid and its derivatives such as methacrylic acid, methyl methacrylate, ethyl methacrylate, butyl methacrylate, 2-ethylhexyl methacrylate, dodecyl methacrylate, 2-hydroxyethyl methacrylate, etc.; dimethylaminoethyl acrylate, dimethylaminoethyl methacrylate and their quaternary derivatives; as well as acrylamide monomers such as acrylamide, methacrylamide, N-methylolacrylamide, N,N-dimethylacrylamide, acrylamido-2-methylpropanesulfonic acid and its sodium salt, etc.; styrene monomers such as styrene, xcex1-styrene, p-styrenesulfonic acid and its sodium or potassium salts; and other monomers such as N-vinylpyrrolidone, etc.; and dienic monomers such as butadiene, isoprene, chloroprene, etc.
The aqueous emulsion of the invention can be typically obtained by the following method. That is, as mentioned hereinabove, the aqueous emulsion of the invention can be obtained by polymerizing a vinyl ester monomer through emulsion polymerization in the presence of (1) a modified PVA containing from 1 to 15 mol % of ethylene units in the molecule and having a degree of hydrolysis of at least 95 mol %, which serves as a dispersant, and (2) at least one polymerization initiator selected from hydrogen peroxide, ammonium persulfate and potassium persulfate in a molar ratio to the vinyl ester monomer of from 0.001 to 0.01, in such a polymerization mode that (3) from 5 to 20% by weight of all the vinyl ester monomer is fed into the reactor in the initial stage of polymerization and the initiator is fed thereinto all at a time in a molar ratio to the initial feed of the vinyl ester monomer of from 0.005 to 0.025.
In producing the aqueous emulsion of the invention, it is important to use at least one polymerization initiator selected from hydrogen peroxide, ammonium persulfate and potassium persulfate. Of these, especially preferred is hydrogen peroxide. In the step of emulsion polymerization in the invention, it is also extremely important to define the molar ratio of the polymerization initiator to the vinyl ester monomer to fall between 0.001 and 0.01, more preferably between 0.002 and 0.007, even more preferably between 0.0025 and 0.005. Though its reason is not clear, using the defined amount of the initiator gives an aqueous emulsion having more improved water resistance, more improved high-temperature viscosity stability and more improved low-temperature storage stability. In case where the molar ratio of the polymerization initiator is smaller than 0.001 as in Example 1 in Japanese Patent Laid-Open No. 81666/1996, it is impossible to obtain the aqueous emulsion of the invention. This is obvious from Comparative Example 5 given hereinunder. On the other hand, in case where the molar ratio of he polymerization initiator is larger than 0.01, it is also impossible to obtain the aqueous emulsion of the invention having good water resistance and good high-temperature viscosity stability. This is obvious from Comparative Example 3 given hereinunder.
As the case may be, the polymerization initiator will be combined with a reducing agent to give a redox system reagent for use in the invention. In that case, in general, hydrogen peroxide will be combined with tartaric acid, L-ascorbic acid, Rongalit or the like; and ammonium or potassium persulfate will be combined with sodium hydrogensulfite, sodium hydrogencarbonate or the like. The amount of the reducing agent is not specifically defined, but may generally fall between 0.05 and 3 equivalents, preferably between 0.1 and 2 equivalents, more preferably between 0.3 and 1.5 equivalents to the polymerization initiator to be combined therewith.
Regarding the mode of adding the polymerization initiator to the reaction system, employed herein is a method of adding it thereto all at a time in the initial stage of polymerization, or that is, adding it in one shot thereto. Concretely, from 5 to 20% by weight of all the vinyl ester monomer to be polymerized is first fed into a reactor in the initial stage of polymerization, and the initiator is fed thereinto all at a time in a molar ratio to the initial feed of the vinyl ester monomer of from 0.005 to 0.025, preferably from 0.008 to 0.020, more preferably from 0.01 to 0.018.
In case where the initial feed ratio of the polymerization initiator is smaller than 0.005, for example, as in Example 1 in Japanese Patent Laid-Open No. 81666/1996, the aqueous emulsion of the invention could not be obtained. This is obvious from Comparative Example 5 mentioned hereinunder.
On the other hand, in case where the initial feed ratio of the polymerization initiator is larger than 0.025, for example, as in Example 1 in Japanese Patent Laid-Open No. 80709/1994, it is also impossible to obtain the aqueous emulsion of the invention having good water resistance and good high-temperature viscosity stability. This is obvious from Comparative Example 6 mentioned hereinunder.
One shot addition of the predetermined amount of the polymerization initiator to the reaction system in the initial stage of polymerization ensures the intended effects as above, enabling stable polymerization of monomers, and the filtration residue from the polymerized emulsion is thereby reduced.
For initial polymerization, the monomer and the polymerization catalyst are added to an aqueous solution of the dispersant, and polymerized at a temperature falling between 50 and 70xc2x0 C., preferably between 55 and 65xc2x0 C., for a period of time falling between 15 and 60 minutes, preferably between 20 and 50 minutes. In the initial stage of polymerization, the monomer is preferably fed into the reactor all at a time.
The amount of the dispersant, ethylene-modified PVA to be added to the reaction system is not specifically defined, but preferably falls between 3 and 20 parts by weight, more preferably between 5 and 15 parts by weight, relative to 100 parts by weight of the monomer. If the amount of the dispersant is smaller than 3 parts by weight or larger than 20 parts by weight, the polymerization stability will be low, and the storage stability of the aqueous emulsion produced will be low.
The initial polymerization shall be terminated when the concentration of the remaining vinyl ester monomer (in terms of % by weight of the polymer produced) has reached at most 10%, preferably at most 5%, more preferably at most 1%. The initial polymerization is followed by final polymerization. Also in the stage of final polymerization, the polymerization catalyst is preferably added to the reaction system all at a time (in a mode of one shot addition), but, as the case may be, it may be added thereto in a continuous or intermittent addition mode. In the final polymerization stage, the monomer is preferably added to the system in a continuous addition mode, but, as the case may be, it may be added thereto all at a time. The polymerization temperature in the final polymerization stage is preferably higher by from 5 to 30xc2x0 C. than that in the initial polymerization stage. Concretely, it may fall between 55 and 100xc2x0 C., preferably between 60 and 95xc2x0 C., more preferably between 70 and 90xc2x0 C.
The polymerization pressure may be normal pressure both for initial polymerization and final polymerization, but, as the case may be, increased pressure will be needed. In particular, in case where copolymers of a vinyl ester with any other comonomer, for example, with ethylene are produced, the monomers must be polymerized under increased pressure.
It is surprising that, in the method of producing an aqueous emulsion as above, which specifically comprises polymerizing a vinyl ester monomer through emulsion polymerization in the presence of a dispersant of a modified PVA containing from 1 to 15 mol % of ethylene units and having a degree of hydrolysis of at least 95 mol % and in the presence of a polymerization initiator in a molar ratio to the vinyl ester monomer of from 0.001 to 0.01, in such a polymerization mode that from 5 to 20% by weight of all the vinyl ester monomer is fed into the reactor in the initial stage of polymerization and the polymerization initiator is fed thereinto all at a time (in one shot) in a molar ratio to the initial feed of the vinyl ester monomer of from 0.005 to 0.025, the polymerization stability is good, and that, according to the method, the aqueous emulsion obtained has the advantages of far improved water resistance and far improved high-temperature viscosity stability (that is, the emulsion is anywhere stable independently of ambient temperatures, and its high-temperature storage stability is good) and has the additional advantage of far improved low-temperature storage stability. This is unexpected at all from the related inventions disclosed in Japanese Patent Laid-Open Nos. 81666/1996, 80709/1994 and 226774/1998. In particular, it is surprising that such excellent aqueous emulsions are obtained in the method of the invention where the amount of the polymerization initiator to be used is specifically defined and the polymerization initiator is added to the system all at a time in the initial stage of polymerization while its amount is specifically defined, and the industrial meaning of the invention is significant to that effect.
The aqueous emulsion of the invention obtained in the manner as above is characterized by its improved water resistance, and is therefore favorable to various applications that require water resistance. In addition, the aqueous emulsion of the invention depends little on ambient temperatures. At elevated temperatures of around 60xc2x0 C., in general, the viscosity of ordinary vinyl ester polymer emulsions containing an ethylene-modified PVA as a dispersion stabilizer increases greatly. As opposed to these, the viscosity of the aqueous emulsion of the invention does not increase so much even at such elevated temperatures. Therefore, the aqueous emulsion of the invention is free from viscosity increase in its production through emulsion polymerization. In addition, while being stored and transported at high temperatures of around 60xc2x0 C. or so, the aqueous emulsion of the invention is protected from viscosity increase, and its workability and handlability is therefore extremely good.
The aqueous emulsion of the invention preferably contains a glycol ether compound. More preferably, a glycol ether compound is added to the system of emulsion polymerization to form the aqueous emulsion. Containing a glycol ether compound, the viscosity ratio T60xc2x0 C./T20xc2x0 C. or H60xc2x0 C./H20xc2x0 C. of the emulsion can be reduced more. In addition, even when its concentration is higher than usually, the emulsion containing a glycol ether compound can be stable and fine.
The glycol ether compound includes, for example, ethylene glycol monomethyl ether, ethylene glycol dimethyl ether, ethylene glycol monoethyl ether, ethylene glycol diethyl ether, ethylene glycol monopropyl ether, ethylene glycol dipropyl ether, ethylene glycol monobutyl ether, ethylene glycol dibutyl ether, propylene glycol monomethyl ether, propylene glycol dimethyl ether, propylene glycol monoethyl ether, propylene glycol diethyl ether, propylene glycol monopropyl ether, propylene glycol dipropyl ether, propylene glycol monobutyl ether, propylene glycol dibutyl ether; and also ethylene glycol condensates such as diethylene glycol monomethyl ether, diethylene glycol dimethyl ether, diethylene glycol monoethyl ether, diethylene glycol diethyl ether, diethylene g glycol monobutyl ether, diethylene glycol dihexyl ether, triethylene glycol monomethyl ether, tetraethylene glycol dimethyl ether, pentaethylene glycol diethyl ether, hexaethylene glycol dipropyl ether; as well as dipropylene glycol monomethyl ether, dipropylene glycol dimethyl ether, tripropylene glycol dimethyl ether, tetrapropylene glycol dimethyl ether, hexapropylene glycol dimethyl ether, diethylene glycol, triethylene glycol, tetraethylene glycol, dipropylene glycol, 3-methoxy-1-butanol, 3-methoxy-3-methyl-1-butanol, etc. Of these, preferred are hydroxyl group-having glycol ether compounds; and especially preferred is 3-methoxy-3-methyl-1-butanol, which is available on the market, for example, as Kuraray""s MMB. Further the glycol ether compound includes glycol ether having a phenyl group, for example, ethylene glycol monophenyl ether, ethylene glycol diphenyl ether, propylene glycol monophenyl ether, polyethylene glycol monophenyl ether. One or more of the compounds may be used either singly or as combined.
The amount of the glycol ether compound to be added to the emulsion is not specifically defined, but may fall generally between 0.5 and 20 parts by weight, preferably between 1 and 10 parts by weight, relative to 100 parts by weight of the solid content of the vinyl ester resin emulsion. If the amount of the compound added is smaller than 0.5 parts by weight, the size of the solid particles constituting the emulsion could not be reduced to a desired degree, and the transparency of the films of the emulsion could not be increased so much. On the other hand, if too much glycol ether compound over 20 parts by weight is added to the polymerization system to give the emulsion, the polymerization stability will be lowered.
The aqueous emulsion of the invention produced according to the method as above may be directly used as it is, but, if desired, it may be combined with any other known emulsions not interfering with the object and the effect of the invention.
The dispersant to be in the aqueous emulsion of the invention is the above-mentioned, ethylene-modified PVA having a degree of hydrolysis of at least 95 mol %. If desired, however, it may be combined with any known anionic, nonionic or cationic surfactants, other PVA, hydroxyethyl cellulose, etc.
As having the excellent characteristics mentioned above, the aqueous emulsion of the invention is favorably used in various fields of paper-processing adhesives for paper or pulp products such as paper tubes, paper bags, paper laminates, corrugated cardboard, etc.; wood-processing adhesives for flush panels, wood laminates, butt-welded wood plates, plywood products, secondary worked plywood products (for jointing them), other ordinary wood products, etc.; adhesives for plastics; binders for dip-coated paper, non-woven fabrics, etc.; as well as admixtures, jointing agents, coating compositions, paper processing agents, fiber processing agents, etc.